Light on the Olympus E-M1 autofocus: S-AF uses phase detection.

Started Oct 10, 2014 | Discussions thread
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photofan1986 Veteran Member • Posts: 3,750
Light on the Olympus E-M1 autofocus: S-AF uses phase detection.

Or does it?

I recently got an Olympus E-M1, and while I was playing with my 45 1.8, I discovered some strange behavior in low light. The camera does not seem very prompt at focusing, hesitating and feeling nervous. Then I made a comparison video between the E-M1, the E-M5 and the GX7.

The GX7 came out on top, as expected. Those Panny cameras are incredible, even in low light. But the surprise was to discover that the E-M5 managed to do much better than his big brother.


Some members criticized the testing procedure, as I am using the big focusing box on the E-M5 and the small one on the E-M1. True enough, but there is no option on the E-M1 to make the "standard" focusing box bigger (and that's really a shame! Why on earth did they get rid of the big focusing box?). On the GX7 you can choose the size of the focusing box and leave it there (even after restart).

So I went on and made the focusing box bigger on the E-M1 through the magnified view.
The situation improved, I think to the level of the E-M5. But then, I tried to make the focusing box smaller, i.e. the size of the "default" focusing box... Surprise! The autofocus behavior is not the same as with the "standard view", even though the AF box size is the same.

What I discovered ?

I performed other tests, and to my big surprise, I discovered that the E-M1 uses phase detection autofocus (probably in conjunction with contrast detection) in S-AF, too!
However, when you use the magnified view, then it switches to contrast detection only.
I remember some time ago, some people were complaining about focus accuracy (don't remember on which camera though) with some long tele lenses. A cure to that was to use the magnified focusing box (green box). That's probably because in the magnified view, the E-M1 switches to contrast detection, which seems always more accurate than phase detection.

How do I know ?

Well, I don't "know" for sure, but I'm pretty much convinced it is the case. Here is why.
Look at this video I just made: you can see that when I point to the subject using the regular focusing box, once the focus is acquired, the camera does not "hunt" any more when trying to focus. That's typically phase detection autofocus. Using the pedestrian Panasonic 20 1.7 on my E-M1, I was surprised that sometimes, the lens didn't even focus, I mean hunt, and the camera snapped away!
And the moment I switch to the magnified green box AF, the camera focuses like a regular contrast detection system: it hunts through the focusing range, and comes back to the point where there is the most contrast.

In this second video, you can see that when the camera is used with the "regular" AF box, the camera seems to "know" where it is going with the focusing. Once it gets close to the ideal point, my guess is that the contrast detection takes the hand. Focusing feels rather "nervous".
With the magnified view, focusing is much smoother, but again, it has to get through the whole focusing scale before coming back to the point with the best contrast: typical contrast detection.

And so what ?

Well, you can conclude whatever you want, but for me, it's now easier to understand why, in some instances, the E-M1 seems worse than my E-M5 at focusing, and sometimes it's better. In good light, the E-M1 snaps into focus very quickly, with no need to get through the focusing range. But in bad light, it seems like Olympus still needs to work on the transition between phase detection and contrast detection. I hope we will get another FW update with this "issue" sorted out.

The good news is that you can, whenever you want to use contrast detection only, switch to the magnified view, and get results that are in all points comparable to the E-M5.

EDIT: all these tests were performed with FW 2.0. Former version might behave differently.

Thanks for reading me, and please share your thoughts!


 photofan1986's gear list:photofan1986's gear list
Fujifilm FinePix F200EXR Sony RX100 III Panasonic Lumix DMC-GM5 Sony a7R III Olympus E-M5 III +16 more
Olympus E-M1
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